Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction becomes problematic. One of the keys to manufacturability of ultra-high density MRAMs is to provide a robust magnetic switching margin by eliminating the half-select disturb issue. For this reason, a new type of device called a spin transfer (spin torque) device was developed. Compared with conventional MRAM, spin-transfer torque or STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. The difference between a STT-MRAM and a conventional MRAM is only in the write operation mechanism. The read mechanism is the same.
Recently, J-G. Zhu et al. described another spintronic device called a spin transfer oscillator in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. A field generator is placed inside the write gap (between the magnetic pole and write shield) to produce a high frequency field in the media. FIG. 1 is taken from the aforementioned reference and shows an ac field assisted perpendicular head design. The upper caption 19 represents a perpendicular spin torque driven oscillator for generating a localized ac field in a microwave frequency regime and includes a bottom electrode 11a, top electrode 11b, perpendicular magnetized reference layer 12 (spin injection layer or SIL), metallic spacer 13, and oscillating stack 14. Oscillator stack 14 is made of a field generation layer (FGL) 14a and a layer with perpendicular anisotropy 14b having an easy axis 14c. The ac field generator in the upper caption 19 is rotated 90 degrees with respect to the lower part of the drawing where the device is positioned between a write pole 17 and a trailing shield 18. The writer moves across the surface of a magnetic media 16 that has a soft underlayer 15. The reference layer 12 provides for spin polarization of injected current (I). Layers 14a, 14b are ferromagnetically exchanged coupled. The media grain can be switched under lower write field due to the assisting AC field. Thus, MAMR is considered to be one of the future technologies which may further improve recording density of perpendicular recording beyond 1 Terabit per square inch.
PMA materials have been considered for MAMR applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is a key challenge for future MRAM applications and other spintronic devices.
A microwave frequency field generator is also called a spin oscillator and is typically made of a multilayer film resembling a current perpendicular to plane (CPP) giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) spin valve. A detailed structure has been described in U.S. Patent Application Publication 2008/0019040. In general, a spin transfer oscillator (STO) includes a non-magnetic spacer sandwiched between a spin injection layer (SIL) and a field generation layer (FGL). The SIL has perpendicular magnetic anisotropy (PMA) and is magnetized in a direction parallel to the down track direction of the write head. When electrons transit the SIL and become polarized, the polarized electrons then pass through the non-magnetic spacer and through the FGL to induce a gyromagnetic oscillation in the FGL.
Higher recording density requires a smaller pole width in the write pole at the air bearing surface (ABS). Since the overlying STO is typically self aligned, the same critical dimension is needed for the STO along the ABS. However, when recording density requires a pole width of about 50 nm or less, a self aligned STO/write pole is difficult to fabricate with the desired profile and shape control. Furthermore, an overhang profile (pole width>leading edge width) is usually generated with existing designs and processes that causes the write pole to break during ion milling.
U.S. Pat. No. 7,333,296 discloses a trapezoidal shaped pole layer formed in a non-magnetic pole encasing layer, and a polishing stopper layer to control the thickness of the pole layer. In U.S. Patent Application 2007/0283557, a bevel angle promotion layer is formed adjacent to the leading edge of a main pole to facilitate the ion milling process. However, neither of the aforementioned references address the issue with breakage during manufacture of a self aligned STO/write pole stack when pole width is less than about 50 nm. Therefore, an improved write pole/STO integrated design is needed to improve shape control and structure reliability for high recording MAMR devices.